Effect of Surrounding Medium on Intramolecular Conformational Changes in Probe Molecules

Conformational dynamics of short probes dispersed in a matrix is investigated. The matrix can be in two states, fast or slow, depending on the amount of free volume. Following the work of Anderson and Ullman,4 the amount of free volume is assumed to fluctuate in time, thus modifying the states of the matrix. The probe undergoes transitions between two states resembling trans-cis isomeric transitions. Correlation times of the probe-matrix system exhibit a clear transition as the free-volume fluctuations of the matrix become faster. In a fast matrix, the temperature dependence of correlation times for probes reflects both the intramolecular conformational barrier in the probe and the energy change of viscous origin in the matrix, while in a slow matrix only the activation energy of the matrix is observed. This is in agreement with the results from recent experiments with fluorescent probes dissolved in small-molecule solvents and in bulk polymer.